The invention disclosed herein is a "subject invention" under NASA Contract. NAS5-33000, Subcontract No. F14000-J19513.
This invention relates to a signal processing system employing orthogonal signal channels with carrier regeneration, such as a QPSK (quadrature phase shift keying) demodulator, operative with an input data signal which is a composite of two orthogonal signal components, the demodulator having inphase and quadrature channels for extraction of inphase and quadrature components of the data. More particularly, the invention relates to a QPSK demodulator having a variable phase offset circuit at an output of an oscillator for adjusting a phase difference between reference carrier components to the two channels to compensate for a deviation in orthogonality of the components of the input data signal.
The Costas loop is a well-known form of a demodulator for reception of a QPSK signal. A basic form of the Costas loop is shown in FIG. 1 of Waters, U.S. Pat. No. 4,344,178. A mathematical description of an input composite signal having inphase and quadrature components is disclosed in Col. 1 of Ryan, U.S. Pat. No. 4,092,606. The incoming QPSK signal, to be processed by the demodulator, is composed of two orthogonal components, namely, an inphase (I) component and a quadrature (Q) component. Mathematically, the two signals may be represented as two sinusoidal waveforms wherein one of the waveforms is a sinusoid and the other is a cosinusoid. Each component is modulated separately with data, by shifting a phase of the component by 180 degrees.
The Costas loop is provided with two signal channels, an I channel and a Q channel. The input signal is split between the two channels with the inphase signal component being extracted by the I channel, and the quadrature signal component being extracted by the Q channel. The two channels provide signals which are combined to provide a loop error signal for driving a voltage controlled oscillator (VCO). The VCO outputs a carrier reference signal for demodulation of the sinusoidal components of the composite input signal. A phase shifter is employed at an output terminal of the oscillator to provide both inphase and quadrature reference carrier signals for use by the I and the Q channels.
The foregoing form of demodulator provides satisfactory operation in the usual situation wherein the inphase and the quadrature signal components are truly orthogonal, that is, wherein the sinusoidal components are shifted in phase by 90 degrees relative to each other. However, there are situations wherein the source of the composite input signal may be somewhat imperfect resulting in the production of a composite signal wherein the two components deviate in their orthogonality. For example, if such a defective composite signal is applied to the foregoing Costas loop, it is observed that it is impossible to align the inphase and the quadrature reference carrier signals with the components of the composite input signal. As a result, some crosstalk is observed in the demodulated signals extracted from the I and the Q channels of the demodulator. The crosstalk is undesirable, and tends to degrade the fidelity of the received data signals.